Amyloid beta peptide (“Aβ”) is a primary component of β amyloid fibrils and plaques, which are regarded playing a role in an increasing number of pathologies. Examples of such pathologies include, but are not limited to, Alzheimer's Disease, Down's syndrome, Parkinson's disease, memory loss (including memory loss associated with Alzheimer's disease and Parkinson's disease), attention deficit symptoms (including attention deficit symptoms associated with Alzheimer's disease (“AD”), Parkinson's disease, and Down's syndrome), dementia (including pre-senile dementia, senile dementia, dementia associated with Alzheimer's disease, Parkinson's disease, and Down's syndrome), progressive supranuclear palsy, cortical basal degeneration, neurodegeneration, olfactory impairment (including olfactory impairment associated with Alzheimer's disease, Parkinson's disease, and Down's syndrome), β-amyloid angiopathy (including cerebral amyloid angiopathy), hereditary cerebral hemorrhage, mild cognitive impairment (“MCI”), glaucoma, amyloidosis, type II diabetes, hemodialysis (β2 microglobulins and complications arising therefrom), neurodegenerative diseases such as scrapie, bovine spongiform encephalitis, and Creutzfeld-Jakob disease and the like.
Aβ peptides are short peptides which are made from the proteolytic break-down of the transmembrane protein called amyloid precursor protein (“APP”). Aβ peptides are made from the cleavage of APP by β-secretase activity at the position corresponding to the N-terminus of Aβ, and by γ-secretase activity at the position corresponding to the C-terminus of Aβ. (APP is also cleaved by α-secretase activity, resulting in the secreted, non-amyloidogenic fragment known as soluble APPα.) Beta site APP Cleaving Enzyme (“BACE-1”) is regarded as the primary aspartyl protease responsible for the production of abnormal Aβ by β-secretase activity. The inhibition of BACE-1 has been shown to inhibit the production of Aβ.
Alzheimer's disease is estimated to afflict more than 20 million people worldwide and is believed to be the most common cause of dementia. AD is a disease characterized by degeneration and loss of neurons and also by the formation of senile plaques and neurofibrillary tangles. Presently, treatment of Alzheimer's disease is limited to the treatment of its symptoms rather than the underlying causes. Symptom-improving agents approved for this purpose include, for example, N-methyl-D-aspartate receptor antagonists such as memantine (Namenda®, Forrest Pharmaceuticals, Inc.), cholinesterase inhibitors such as donepezil (Aricept®, Pfizer), rivastigmine (Exelon®, Novartis), galantamine (Razadyne Reminyl®), and tacrine (Cognex®).
In AD, Aβ peptides, formed through β-secretase and γ-secretase activity, can form tertiary structures that aggregate to form amyloid fibrils. Aβ peptides have also been shown to form Aβ oligomers (sometimes referred to as “Abeta aggretates” or “Abeta oligomers”). Aβ oligomers are small multimeric structures composed of 2 to 12 Aβ peptides that are structurally distinct from Aβ fibrils. Amyloid fibrils can deposit outside neurons in dense formations known as senile plaques, neuritic plaques, or diffuse plaques in regions of the brain important to memory and cognition. Aβ oligomers are cytotoxic when injected in the brains of rats or in cell culture. This Aβ plaque formation and deposition and/or Aβ oligomer formation, and the resultant neuronal death and cognitive impairment, are among the hallmarks of AD pathophysiology. Other hallmarks of AD pathophysiology include intracellular neurofibrillary tangles comprised of abnormally phosphorylated tau protein, and neuroinflammation.
Evidence suggests that Aβ and Aβ fibrils and plaque play a causal role in AD pathophysiology. (See Ohno et al., Neurobiology of Disease, No. 26 (2007), 134-145.) Mutations in the genes for APP and presenilins 1/2 (PS1/2) are known to cause familial AD and an increase in the production of the 42-amino acid form of Aβ is regarded as causative. Aβ has been shown to be neurotoxic in culture and in vivo. For example, when injected into the brains of aged primates, fibrillar Aβ causes neuron cell death around the injection site. Other direct and circumstantial evidence of the role of Aβ in Alzheimer etiology has also been published.
BACE-1 has become an accepted therapeutic target for the treatment of Alzheimer's disease. For example, McConlogue et al., J. Bio. Chem., vol. 282, No. 36 (September 2007), have shown that partial reductions of BACE-1 enzyme activity and concomitant reductions of Aβ levels lead to a dramatic inhibition of Aβ-driven AD-like pathology (while minimizing side effects of full inhibition), making β-secretase a target for therapeutic intervention in AD. Ohno et al. Neurobiology of Disease, No. 26 (2007), 134-145, report that genetic deletion of BACE-1 in 5XFAD mice abrogates Aβ generation, blocks amyloid deposition, prevents neuron loss found in the cerebral cortex and subiculum (brain regions manifesting the most severe amyloidosis in 5XFAD mice), and rescues memory deficits in 5XFAD mice. The group also reports that Aβ is ultimately responsible for neuron death in AD and conclude that BACE-1 inhibition has been validated as an approach for the treatment of AD. Roberds et al., Human Mol. Genetics, 2001, Vol. 10, No. 12, 1317-1324, established that inhibition or loss of β-secretase activity produces no profound phenotypic defects while inducing a concomitant reduction in β-amyloid peptide. Luo et al., Nature Neuroscience, vol. 4, no. 3, March 2001, report that mice deficient in BACE-1 have normal phenotype and abolished β-amyloid generation.
BACE-1 has also been identified or implicated as a therapeutic target for a number of other diverse pathologies in which Aβ or Aβ fragments have been identified to play a role. One such example is in the treatment of AD-type symptoms of patients with Down's syndrome. The gene encoding APP is found on chromosome 21, which is also the chromosome found as an extra copy in Down's syndrome. Down's syndrome patients tend to acquire AD at an early age, with almost all those over 40 years of age showing Alzheimer's-type pathology. This is thought to be due to the extra copy of the APP gene found in these patients, which leads to overexpression of APP and therefore to increased levels of Aβ causing the prevalence of AD seen in this population. Furthermore, Down's patients who have a duplication of a small region of chromosome 21 that does not include the APP gene do not develop AD pathology. Thus, it is thought that inhibitors of BACE-1 may be useful in reducing Alzheimer's type pathology in Down's syndrome patients.
Another example is in the treatment of glaucoma (Guo et al., PNAS, vol. 104, no. 33, Aug. 14, 2007). Glaucoma is a retinal disease of the eye and a major cause of irreversible blindness worldwide. Guo et al. report that Aβ colocalizes with apoptotic retinal ganglion cells (RGCs) in experimental glaucoma and induces significant RGC cell loss in vivo in a dose- and time-dependent manner. The group report having demonstrated that targeting different components of the Aβ formation and aggregation pathway, including inhibition of β-secretase alone and together with other approaches, can effectively reduce glaucomatous RGC apoptosis in vivo. Thus, the reduction of Aβ production by the inhibition of BACE-1 could be useful, alone or in combination with other approaches, for the treatment of glaucoma.
Another example is in the treatment of olfactory impairment. Getchell et al., Neurobiology of Aging, 24 (2003), 663-673, have observed that the olfactory epithelium, a neuroepithelium that lines the posterior-dorsal region of the nasal cavity, exhibits many of the same pathological changes found in the brains of AD patients, including deposits of Aβ, the presence of hyperphosphorylated tau protein, and dystrophic neurites among others. Other evidence in this connection has been reported by Bacon A W, et al., Ann NY Acad Sci 2002; 855:723-31; Crino P B, Martin J A, Hill W D, et al., Ann Otol Rhinol Laryngol, 1995; 104:655-61; Davies D C, et al., Neurobiol Aging, 1993; 14:353-7; Devanand D P, et al., Am J Psychiatr, 2000; 157:1399-405; and Doty R L, et al., Brain Res Bull, 1987; 18:597-600. It is reasonable to suggest that addressing such changes by reduction of Aβ by inhibition of BACE-1 could help to restore olfactory sensitivity in patients with AD.
Other diverse pathologies characterized by the inappropriate formation and deposition of Aβ or fragments thereof, and/or by the presence of amyloid fibrils, include neurodegenerative diseases such as scrapie, bovine spongiform encephalitis, Creutzfeld-Jakob disease and the like, type II diabetes (which is characterized by the localized accumulation of cytotoxic amyloid fibrils in the insulin producing cells of the pancreas), and amyloid angiopathy. In this regard reference can be made to the patent literature. For example, Kong et al., US2008/0015180, disclose methods and compositions for treating amyloidosis with agents that inhibit Aβ peptide formation. Another example is the treatment of traumatic brain injury. As another example, Loane, et al. report the targeting of amyloid precursor protein secretases as therapeutic targets for traumatic brain injury. (Loane et al., “Amyloid precursor protein secretases as therapeutic targets for traumatic brain injury”, Nature Medicine, Advance Online Publication, published online Mar. 15, 2009.) Still other diverse pathologies characterized by the inappropriate formation and deposition of Aβ or fragments thereof, and/or by the presence of amyloid fibrils, and/or for which inhibitor(s) of BACE-1 is expected to be of therapeutic value are discussed further hereinbelow.
The therapeutic potential of inhibiting the deposition of Aβ has motivated many groups to characterize BACE-1 and to identify inhibitors of BACE-1 and of other secretase enzyme inhibitors. Examples from the patent literature are growing and include US2005/0282826, WO2006009653, WO2007005404, WO2007005366, WO2007038271, WO2007016012, US2007072925, WO2007149033, WO2007145568, WO2007145569, WO2007145570, WO2007145571, WO2007114771, US20070299087, US2007/0287692, WO2005/016876, WO2005/014540, WO2005/058311, WO2006/065277, WO2006/014762, WO2006/014944, WO2006/138195, WO2006/138264, WO2006/138192, WO2006/138217, WO2007/050721, WO2007/053506, WO2007/146225, WO2006/138230, WO2006/138265, WO2006/138266, WO2007/053506, WO2007/146225, WO2008/073365, WO2008/073370, WO2008/103351, US2009/041201, US2009/041202, WO2009/131975, WO2009091016, and WO2010/047372.
BACE inhibitors, particularly BACE-2 inhibitors, are an art-recognized target for the treatment of diabetes. Type 2 diabetes (T2D) is caused by insulin resistance and inadequate insulin secretion from the pancreatic beta-cells, leading to poor blood-glucose control and hyperglycemia (M Prentki & C J Nolan, “Islet beta-cell failure in type 2 diabetes.” J. Clin. Investig., 2006, 116(7), 1802-1812). Patients with T2D have an increased risk of microvascular and macrovascular disease and a range of related complications including diabetic neuropathy, retinopathy, and cardiovascular disease.
Beta-cell failure and consequent dramatic decline in insulin secretion and hyperglycemia marks the onset of T2D M Prentki & C J Nolan, “Islet beta-cell failure in type 2 diabetes.” J. Clin. Investig., 2006, 116(7), 1802-1812). Most current treatments do not prevent the loss of beta-cell mass characterizing overt T2D. However, recent developments with GLP-1 analogues, gastrin and other agents show that prevention and proliferation of beta-cells is possible to achieve, leading to an improved glucose tolerance and slower progression to overt T2D. (L L. Baggio & D J. Drucker, “Therapeutic approaches to preserve islet mass in type 2 diabetes”, Annu. Rev. Med. 2006, 57, 265-281.)
Tmem27 has been identified as a protein promoting beta-cell proliferation (P. Akpinar, S. Juqajima, J. Krutzfeldt, M. Stoffel, “Tmem27: A cleaved and shed plasma membrane protein that stimulates pancreatic beta-cell proliferation”, Cell. Metab. 2005, 2, 385-397) and insulin secretion (K. Fukui, Q. Yang, Y. Cao, N. Takahashi et al., “The HNF-1 target Collectrin controls insulin exocytosis by SNARE complex formation”, Cell. Metab. 2005, 2, 373-384.) Tmem27 is a 42 kDa membrane glycoprotein which is a constitutively shed from the surface of beta-cells, resulting from a degradation of the full-length cellular Tmem27. Over expression of Tmem27 in a transgenic mouse increases beta-cell mass and improves glucose tolerance in a DIO model of diabetes. (P. Akpinar, S. Juqajima, J. Krutzfeldt, M. Stoffel, “Tmem27: A cleaved and shed plasma membrane protein that stimulates pancreatic beta-cell proliferation”, Cell. Metab. 2005, 2, 385-397; (K. Fukui, Q. Yang, Y. Cao, N. Takahashi et al., “The HNF-1 target Collectrin controls insulin exocytosis by SNARE complex formation”, Cell. Metab. 2005, 2, 373-384.) Furthermore, siRNA knockout of Tmem27 in a rodent beta-cell proliferation assay (e.g., using INS1e cells) reduces the proliferation rate, indicating a role for Tmem27 in control of beta-cell mass.
In vitro, BACE-2 (but reportedly not BACE-1) cleaves a peptide based on the sequence of Tmem27. BACE-2 is a membrane-bound aspartyl protease and is colocalized with Tmem27 in rodent pancreatic beta-cells (G. Finzi, F. Franzi, C. Placidi, F. Acquati, et al., “BACE-2 is stored in secretory granules of mouse and rat pancreatic beta cells”, Ultrastruct. Pathol. 2008, 32(6), 246-251). It is also known to be capable of degrading APP (I. Hussain, D. Powell, D. Howlett, G. Chapman, et al., “ASP1 (BACE2) cleaves the amyloid precursor protein at the beta-secretase site”, Mol. Cell. Neurosci. 2000, 16, 609-619), IL-1 R2 (P. Kuhn, E. Marjaux, A. Imhof, B. De Strooper, et al., “Regulated intramembrane proteolysis of the interleukin-1 receitpro II by alpha-, beta-, and gamma-secretase”, J. Biol. Chem., 2007, 282(16), 11982-11995). Inhibition of BACE-2 is therefore proposed as a treatment for T2D with the potential to preserve and restore beta-cell mass and stimulate insulin secretion in pre-diabetic and diabetic patients. See, e.g., WO2010128058.